RFID or radio frequency identification technology has been used in a variety of commercial applications such as inventory tracking and highway toll tags. In general, a transceiver tag or transponder transmits stored data by backscattering varying amounts of an electromagnetic field generated by an RFID reader. The RFID tag may be a passive device that derives its electrical energy from the received electromagnetic field or may be an active device that incorporates its own power source. The backscattered energy is then read by the RFID reader and the data is extracted therefrom.
The RFID reader includes a transmitter that provides the electrical energy or information to the RFID tag. To accomplish this, the transmitter employs a power amplifier to drive an antenna with an unmodulated or modulated output signal. Traditionally, in order to generate highly controlled (i.e., shaping the modulation wave in order to minimize unwanted spectral content) amplitude modulation (AM) for the output signal, a highly linear power amplifier running in Class-A mode has been used. However, RFID readers that utilize Class-A power amplifiers are inefficient, require more of heat-sinking, and have poor noise figure. Additionally, these readers are not operable under certain applications such as Power Over Ethernet (POE) which have maximum power consumption requirements.
Various methods have been used to control the power output of the RFID reader. Many of them involve calibrating each individual power output setting step during the reader production process. This requires complex algorithms or lookup tables and time consuming calibration procedures. What is needed is a method for controlling the power output of the RFID reader that allows for accurate steps in the power output setting without requiring large firmware overhead.